MIKROE MRAM Click features MRAM module which contains 262,144 magnetoresistive memory cells, organized into 32,768 bytes of memory. It means that MRAM Click is a memory storage device with 32KB of memory space. The used memory module can withstand an unlimited number of write cycles, it has data retention period greater than 20 years and it can read and write to random addresses with no delay.

Besides these features that already mark this kind of memory modules a milestone in memory manufacturing technology, the device features data protection in case of power loss, block write protection, low power consumption and fast SPI interface that can theoretically work up to 40MHz. Because of its universal memory characteristics, MRAM click can be used both as a non-volatile storage media, or temporary RAM expansion for storing variables in any embedded application.

MRAM is an abbreviation for Magnetoresistive Random Access Memory. This is a fairly new type of memory, first introduced in the ’90s. The main features of MRAM modules include unlimited write endurance, read and write cycles with no delay and data retention even after power loss. This makes these modules a kind of universal memory, a hybrid between standard RAM modules and EEPROM and similar types of FLASH memory. The ability to randomly access the memory cells without the need to refresh them and with no delay makes this type of memory looks like an SRAM. Data retention after power off, makes it look like a FLASH memory. Therefore, it is often referred to as the Universal memory.

There are certain downsides and flaws that prevent this type of memory to become the main memory medium and replace all types of memory used so far. It is the cell density achievable with this technology. Unlike the memory modules used so far which store data as the electrical charge, this type of memory uses magnetic elements to store data. Two plates are used - one is a permanent magnet, while the other plate can be magnetized by a current flow through the conductors and the resulting induced magnetic field. The problem with this is obvious: it is impossible to contain the induced magnetic field in a space small enough to allow for very high cell densities. There are various methods used to overcome this problem, but it remains the main reason why this type of memory hasn’t replaced other memory types, yet. However, in the embedded world, where the memory amount demands are rather limited, this type of memory most certainly deserves attention.

MRAM Click board™ uses the MR25H256, a 256 kilobits serial SPI MRAM memory module from Everspin company. This module contains 262,144 bits of memory that can be randomly accessed. The pinout of the used memory module is the same as most commonly used EEPROM modules so that it can directly replace it. The usual SPI lines - SO, SI, SCK and #CS pins from the MR25H256 IC are routed to the mikroBUS™ SPI port (MISO, MOSI, SCK and CS pins). Besides the SPI serial bus, there are two more pins routed to the mikroBUS™.

The #HOLD pin of the MR25H256 IC is routed to the INT pin of the mikroBUS™ and it is used to hold the data transfer. When this pin is pulled to a LOW logic level, all data transfer operations are suspended. However, this function is enabled only when the device is already addressed with the CS pin pulled to a LOW level. This allows to pause the data transfer and resume it later without the need to first address it via the CS pin, reducing the output latency that way. While the data transfer is paused, the SO pin will switch to a high impedance mode (HIGH Z) and will remain inactive. The SCK pulses are completely ignored. The #HOLD pin of the MR25H256 IC is pulled to a HIGH logic level by an onboard pull-up resistor.

The #WP pin of the MR25H256 IC is routed to the INT pin of the mikroBUS™ and it is used to prevent writes to the status register, acting as a hardware write protect pin. It is routed to the RST pin of the mikroBUS™.

The logical organization of the moduke, such as read and write commands and the status register of the MR25H256 IC are the same as with most commonly used EEPROM modules, such as the one used in EEPROM 4 click. That allows this memory module, as well as MRAM click to replace the existing EEPROM module with not too much additional work. The provided libraries offer all the functions needed to work with the MRAM click. Their usage is demonstrated in the included example application which can be used as a reference for further development. The device should wait for the system voltage to become stable before the writing is attempted. The table below contains write inhibit voltage threshold, as well as the startup time that has to expire for the device to become fully operational. The board is powered only by the 3.3V rail of the mikroBUS™.

• Interface: GPIO, SPI
• Compatibility: mikroBUS™
• Dimensions: 28.6 x 25.4mm
• Input Voltage: 3.3V
• SCK Clock Frequency: Min. 0MHz, Max. 40MHz
• Write Inhibit Voltage Threshold: Min. 2.2V, Max. 2.7V
• Startup Timer: 400uS

• Schematic
• MR25H256A Datasheet
• mikroSDK
• GitHub